Abstract
African swine fever (ASF) has gained panzootic dimensions and commercial vaccines are still unavailable. Recently, a series of live attenuated vaccines has raised hope for an efficacious and safe vaccine, among them "ASFV-G-∆MGF". We tested the latter in an in vivo reversion to virulence study in accordance with International Cooperation on Harmonisation of Technical Requirements for Registration of Veterinary Medicinal Products guidelines. Upon forced animal passaging, a virus variant emerged that was associated with transient fever and an increased replication and shedding. However, all animals were healthy upon completion of the study and reversion to significant virulence was not observed. The genomic changes did not affect the recombination site but involved deletions and reorganizations in the terminal regions of the genome. Thus, our study underscores that in-depth safety characterization is needed for live ASF vaccines. For this particular candidate, additional studies should target long-term effects and transmission characteristics before thorough benefit-risk analysis can be carried out.